Electron microscope



Feb. 3, 1942. E. G. RAMBERG ETAL 2,271,990

ELECTRON MICROSCOP Filed July 29, 1939 Snvcntol Patented Feb. 3, 1942 ELECTRON MICROSCOPE Haddon Heights, and George A. Morton, Audubon, N. J., assignors to Radio Corporation of America, a corporation of Edward G. Bamberg,

Delaware Application July 29, 1939, Serial No. 287,380

(Cl. Z50-49.5)

7 Claims.

This invention relates to electron discharge devices, particularly to means for producing images of objects and has special reference to the provision of improvements in electron microscopes.

It may be said generally that electron microscopes can be made to have a greater resolving power (due to the shortness of the wave lengths of the electrons) than ordinary optical microscopes. This advantage coupled with the relatively enormous useful magnification of which electron microscopes are capable recommends their use in the study of metals and other inorganic and organic substances. In fact many valuable advances have been made in various arts as the result of the use of such microscopes.

However, electron microscopes have not enjoyed the universal acceptance which they apparently merit. This is so principally because such devices have heretofore incorporated electron-lens systems which, irrespective of type (e. g. electromagnetic or electrostatic) render their design, structure and use far more complicated than is desirable in the case of an instrument Whose use may be entrusted to one possibly unfamiliar with complex electrical apparatus. Further, the images produced in the electron microscopes of the prior art have suiered Vfrom various image defects resulting from the heretofore necessary presence of such electron-lens f'i systems. While various auxiliary focusing elements (such as electron Velocity filters, electron mirrors, etc.) have heretofore been employed in compensating for certain of these defects, other defects in the image (e. g. aperture defects) cannot be compensated for by any presently known means, however complicated.

Accordingly, the principal object of thepresent invention is to provide an electron image device which is operable to produce a magnified image of a specimen or other object Without the use of an electron lens system for focusing the electrons either upon the object or upon the viewing screen or other target.

Another and important object of the invention is to provide an electron microscope wherein the resolution is limited (apart from chromatic aberrations due to variations in the initial velocities of the electrons) only by Fresnel diffraction adjacent the specimen.

Another object of the invention is to provide an electron microscope capable of various desired degrees of magnification and wherein the image is at all times in focus irrespective of the degree of magnification selected.

Another object of the invention is to provide an electron image tube which is insensitive to voltage variations (such as may be caused by fluctuations originating in the supply mains) and other irregularities (such as variations in the emissive characteristic of the cathode) which have heretofore resulted in defects in the image` The foregoing objects are achieved in accordance with the invention by the substitution of a point cathode and an electrode for creating an electrostatic eld capable of maintaining a cold discharge of electrons from said point in place of (o.) the usual thermionic or photosensitive cathode and (b) the several magnets and/or focusing rings, cylinders or plates heretofore employed in apparatus of the general character described. The elimination of the electromagnetic and/or electrostatic lens systems of the prior art is achieved without any sacrice, and indeed a positive gain, in the quality of the image.

The novel features characteristic of the invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and its method of operation, together with additional objects and advantages, will be best understood from the following description of certain specic embodiments thereof, when read in connection with the accompanying drawing, in which Figure 1 is a longitudinal sectional view of a high vacuum discharge tube embodying the invention; and

Figure 2 is a similar view of an alternative embodiment of the invention.

Like reference characters designate the same or corresponding parts in both figures.

Referring particularly to Fig. l, I designates generally a tube or envelope which may be of substantially any desired shape and dimension and which, in the instant case, is provided with a transparent window 2 coated on the inside with Willemite, zinc sulphide or like fluorescent substance 3. The envelope I will be understood to be highly evacuated, say to a pressure of the order of 10-5 mm. of mercury.

Mounted along the central axis of the tubes as on a press 4, is a cold point-cathode which, as shown in Fig. l and Fig. 2, may comprise a needle-like element or tapered rod 5 whose point is directed toward the target or screen 3. The inner surface of the neck of the tube may be shielded against cold emission from the body or shank of the cathode as by means of an auX- iliary structure which is exemplied in the drawing by a cylinder 6 which surroimds the cathode through the greater portion of its length and which is supported thereon as by means of a pair of apertured discs 'I'. The supports 'I may be of metal, since it is usually desirable to maintain the shield at cathode potential. The free end or point P of the cathode preferably extends slight ly beyondv the terminal of the shield 6.

In order to initiate and maintain a flow of electrons from the point of the cathode 5, it is necessary to provide an auxiliary electrode which isy maintained at a potential considerably higher (say, 5000 volts higher) than the cathode. The spacing between the cathode and such auxiliary or discharge maintaining electrode is by no means critical. It may, for example, be spaced several inches from the cathode or it may be spaced closely adjacent thereto. In the illustrated embodiment of the invention, member 8 serves not only as the discharge maintaining electrode but also comprises a support for the object or specimen 9 to be magnified. Electrode 8 may comprise a disc (see Fig. 1) having a central aperture I0, in line with the point of the cathode, and within which the specimen to be examined is supported. Alternatively it may be in the form of a grid (8a, Fig. 2) in which case the specimen may be mounted in one of its. centrally located interstices. The electrode 3 may conveniently be supported upon a frame comprising a pair of rods I I and I2, which in the illustrated embodiment of the invention is secured to the press 4, and is provided with an external lead I3 which is connected to a point on the direct current source I4 which is several thousand volts more positive than the point to which the cathode lead I5 is. connected.

Another electrode I6 which may conveniently be in. the form of a circular metallic deposit on the inner wall of the tube is the only other electrode in the device of Fig. 1. This electrode I6 may comprise the discharge maintaining electrode but in the instant case is employed as a collector for such secondary electrons as are emitted by reason of the impact of the primary electrons upon the screen 2. In operating the device of Fig. 1, the electrode I6 when employed as a collector of secondary electrons may be maintained at the same or slightly diiTerent (preferably lower) potential than the electrode 8, which latter electrode, as above indicated, may here be considered to be the discharge maintaining electrode.

The magniication of the device of Fig. 1 is xed and is not subject to adjustment. Magnification is determined by the formula m=b'/a where a designates the distance from the center of curvature of the point of the cathode to the specimen (il-Figs. 1 and 2) and b is the distance from the center of curvature of the point to the viewing screen 3.

Since the radius of curvature of the cathode point may be of the order of -5 inches and the distance between this point, or the specimen 9 (which may be within a few one-hundred thousandths of an inch of the cathode), and the screen may be inches (more or less, as desired) it is apparent that magnification of the order of 1,000,000 diameters can, in principle, be achieved. In practice, the size of the specimen is the primary factor inhibiting the achievement of this maximum degree of magnication.

It will be seen from the above formula that the magnification of the device of Fig. 1 will be altered if the distance a can be varied. To this end the device of Fig. 2 embodies means for varying the position of the specimen with respect to the point of the cathode. In its simplest form the means for moving the mount (8a) for the specimen may comprise a pair of bimetallic elements 2l and 22 formed of inner strip a of nickel and an outer strip b of tantalum which are incorporated respectively in the opposite arms II and I2 of the frame which supports the mount 8a. These elements are dimensioned to present an appreciable resistance to the now of current.

' Thus, when a heating current is applied thereto through the leads 23 and 24 the mount 8a and hence the specimen 9 thereon will be moved toward the cathode 5 (the relative arrangement of the bimetal strips permitting this) and the magnication will be increased.

As a matter of practice, it has been found desirable to incorporate a second pair of bimetallic elements 25, 26 in series with the elements 2| and 22. As indicated at b, a, the metal strips. comprising the bimetallic elements 25, 26 have an opposite arrangement, that is to say, the tantalum strip is on the inside and the nickel strip on the outside. Thus, during the baking process to which the tube is subject during its manufacture, all ofy the bimetallic elements 2|, 22, 25, 26 expand to the same extent but in opposite directions, so that at that time the position of the support 8 is not changed with respect to the cathode. These auxiliary bimetal strips are dimensioned to present a substantially negligiblel resistance to the flow of the heating current to the resistive elements 2|, 22.

It is sometimes desirable to outgas the cathode prior to operating the device (but after it has been constructed) and for this purpose an auxiliary heater electrode 21 may be connected to the cathode as by looping it thereabouts near its pointed end, preferably within the space circumscribed by the shield 6.

It will be apparent to those skilled in the art that the envelope of the electron microscope of the invention may be made in demountable sections, land that the specimen may be removably mounted in the tube by the provision of a suitable airlock, as, has been proposed in connection with the tubes of the prior art. It will likewise be apparent that the fluorescent screen 3 may be replaced by a photographic film in a suitable holder therefor (not shown), as has also been previously proposed.

Other modifications of the invention such, for example, as the substitution of a mount which is movable by an externally applied magnetic field, instead of by the described thermally actuated means., will suggest themselves to those skilled in the art. Accordingly, it is to be understood that the foregoing description of certain of the numerous possible embodiments of the invention is to be interpreted as illustrative and not in a limiting sense except as required by the prior art and by the spirit of the appended claims.

What is claimed is: f

1. In an electron discharge device comprising an evacuated envelope containing a cathode terminating in a point comprising a non-thermionic source of electrons, and means mounted in spaced relation with respect to said cathode and surrounding said point for heating said cathode, whereby to out-gas it.

2. In an electron microscope comprising an evacuated envelope containing a cathode terminating in a point comprising a source of electrons, an electrode mounted in spaced relation with respect to said cathode, said cathode and said electrode being adapted to have an electrostatic field created therebetween for maintaining a discharge oi electrons from said point, means including said electrode for supporting a specimen in the path of said electrons, and means operatively connected to said electrode for moving Said specimen in said path with respect to said point.

3. In an electron microscope comprising an evacuated envelope containing a cathode terminating in a point comprising a source of electrons, an/electrode mounted in spaced relation with respect to nsaid cathode, said cathode and said electrode being adapted to have an electrostatic eld created therebetween for maintaining a discharge oi electrons from said point, and means adapted to support a specimen closely adjacent said point, said electrostatic field constituting the sole means for directing said electrons from said point to said specimen.

4. The invention as set forth in claim 3 and wherein said discharge maintaining electrode comprises said specimen supporting means.

5. An electron microscope comprising an evacuated envelope containing a cathode terminating in a point comprising a source of electrons, a viewing device mounted in spaced relation with respect to said cathode and toward which said point is directed, an electrode mounted in the space between said cathode and said viewing device, said cathode and said electrode being adapted to have an electrostatic eld created therebetween for maintaining a discharge of electrons from said point, and means adapted to support a specimen closely adjacent said point in the path of said electrons, the electrostatic field created between said cathode and said electrode constituting the sole means for directing said electrons from said point to said specimen and thence from said specimen to said viewing device.

6. An electron microscope comprising an evacuated envelope containing adjacent one end a cathode terminating in a point comprising a non-thermionic source of electrons, a fluorescent screen mounted adjacent the opposite end of said envelope and toward which said point of said cathode is directed, an electrode mounted in the space between said opposite ends of said envelope and comprising a support for a specimen to be examined, said cathode and said electrode being adapted to have an electrostatic eld created therebetween for creating a cold discharge of electrons from said point along a path which includes said specimen, the electrostatic field created between said cathode and said electrode constituting the sole means for projecting an electron image-of said specimen upon said fluorescent screen.

7. An electron microscope comprising a highly evacuated envelope containing a cathode terminating in a point comprising a source of prirmary electrons, an electrode mounted in spaced relation with respect to said cathode, said cathode and said electrode being adapted to have an electrostatic eld created therebetween for maintaining a discharge of primary electrons from said point, means including said electrode for supporting a specimen in the path of said primary electrons closely adjacent said point, a target electrode toward which said point is directed and upon which said primary electrons eventually impinge, and a collector electrode for collecting secondary electrons resulting from the impact of said primary electrons upon said target electrode, the electrostatic eld created between said cathode and said discharge -maintaining electrode constituting the sole means for establishing the path of said primary electrons within said envelope.

EDWARD G. BAMBERG. GEORGE A. MORTON. 

